The present invention relates to the art of medical diagnostic imaging. It finds particular application in conjunction with magnetic resonance imaging (MRI), and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other like applications.
In MRI, a substantially uniform temporally constant main magnetic field, B.sub.0, is generated within an examination region. The main magnetic field polarizes the nuclear spin system of a subject being imaged within the examination region. Magnetic resonance is excited in dipoles which align with the magnetic field by transmitting radio frequency (RF) excitation signals into the examination region. Specifically, RF pulses transmitted via an RF coil assembly tip the dipoles out of alignment with the main magnetic field and cause a macroscopic magnetic moment vector to precess around an axis parallel to the main magnetic field. The precessing magnetic moment, in turn, generates a corresponding RF magnetic resonance signal as it relaxes and returns to its former state of alignment with the main magnetic field. The RF magnetic resonance signal is received by the RF coil assembly, and from received signals, an image representation is reconstructed for display on a human viewable display.
The appropriate frequency for exciting resonance in selected dipoles is governed by the Larmor equation. That is to say, the precession frequency of a dipole in a magnetic field, and hence the appropriate frequency for exciting resonance in that dipole, is a product of the gyromagnetic ratio .gamma. of the dipole and the strength of the magnetic field. In a 1.5 T magnetic field, hydrogen (.sup.1 H) dipoles have a resonance frequency of approximately 64 MHz. Generally in MRI, the hydrogen species is excited because of its abundance and because it yields a strong MR signal. As a result, typical MRI apparatus are equipped with built-in whole-body RF coils tuned to the resonant frequency for hydrogen.
One obstacle to overcome in MRI is potential degradation in the image reconstruction due a low signal to noise ratio (SNR) in the acquired MR signals or echos.
Previously developed methods employed to address the SNR problem have had generally limited success due to various drawbacks. Some multi-echo sequences decrease the bandwidth of data collection for each echo in succession such that the later echos, which are of a lower amplitude due to MR signal decay, are collected at a lower bandwidth to preserve SNR as best as possible. One example is multi-echo spin echo sequences which provide different contrasts in a set of images built each in turn from one of the echos. However, these techniques do not vary the bandwidth of data collection within a single image. Additionally, the SNR remains degraded by the noise accompanying data from the corners of k-space which are not fully visualized in the image.
Normally, a number of rows or horizontal lines with a predetermined number of sample points are collected for an MR image. This raw data often fills k-space in a rectangular or square shape. Employment of a circular Fermi filter applied to the raw MR data to chop off the corners of k-space has been shown to improve SNR by as much as 13%. However, the disadvantage associated with this technique is that resources are wasted on the collection of data which is ultimately discarded.
There is also the variable encoding time (VET) method. Recognizing that central lines of k-space generally employ less phase encoding as compared to the outer lines, the VET method shortens the inter-echo spacing of a sequence while varying the data sampling window. By shortening the data sampling window time and preserving the same sampling bandwidth, time is made available for larger phase encode lobes, or the time is made available for more data samples when the desired phase encode amounts are low. This allows for the trimming of k-space corners.
The present invention contemplates a new and improved data collection technique which overcomes the above-referenced problems and others.